In the flight vehicle art, many propeller driven and circularly shaped vehicles have been tested, but they all require apparatus in addition and independent of the flight generating apparatus to control the flight of the vehicle. One of the most troublesome problems with the prior art designs is the nose-up pitching moment created during forward flight of the vehicle, and which must be compensated for. This nose-up moment encountered by the prior art was a limiting factor in these vehicles.
An example of this prior art is the Hiller Flying Platform. The Hiller design incorporates two counter-rotating propellers surrounded by a shroud. The propellers were set at a fixed pitch and the amount of lift the propellers generated was controlled by the rotational speed of the propellers. The platform was stabilized and controlled in forward flight by a man leaning back and forth thereby creating a moment about the center of gravity of the platform. In later designs, flow vanes were installed below the propellers to act as additional control surfaces.
Another example of the prior art is AROD, Airborne Remotely Operated Device. AROD incorporates a single rigid propeller in a shroud. Torque to counteract the propeller torque is obtained by placing flow vanes below the propeller. These vanes are movable and deflected proportionally to the amount of anti-torque required. These vanes were also used to control the vehicle in both pitch and roll. Because AROD used only a single propeller, gyroscopic coupling of aircraft pitch and roll existed, and a cross rate feedback control system was required to stabilize the air vehicle.
While Sikorsky Aircraft has used counterrotating rotors, such rotors were unshrouded. Further, while Aerospatiale has used a single shrouded fan as an anti-torque tail rotor for a helicopter, that single shrouded fan had no cyclic pitch control.